Color is a perception, just like taste. Let’s consider an analogy with food. When we eat, our taste buds sense the attributes of food: sweet, salty, sour, or bitter. Point at the foods below to see different mixtures of taste attributes.
Similarly, when we look at a scene, our visual nerves sense the attributes of color — which are: the amount of green-or-red, the amout of blue-or-yellow, and the brightness. To see how these attributes combine to make colors, point at the painting below.
Note that these attributes are opposites, like hot and cold. Color nerves sense green or red; and blue or yellow — not both. Thus, we never see bluish-yellows or reddish-green. The fact that these colors are opposite each other is the foundation of how color works.
Ewald Hering (1834-1918) devised the first accurate theory of color vision. He theorized the "opponent" nature of green/red and blue/yellow. Hering expanded on the ideas of Goethe and Schoepenhauer.
Color attributes were first understood by 19th century from physiologist Ewald Hering. He made the color charts below. His charts show how all colors arrise from a comination of green-or-red, blue-or-yellow, and brightness. (In his diagram, the green-neutral-red is vertical, and blue-neutral-yellow is horizontal.) The left circle shows relative mixtures of color attributes. The right circle shows what we perceive when these attributes are mixed.
We can understand much about color by considering how the green-red and blue-yellow color attributes interact. For example, contrasting colors are are on opposite edges of the color circle.
A modern representation of color space. This is conceptually similar to Hering’s circles, but the middle fades to gray.
The artistic term "hue" is the edges of the square (the outside of Hering’s circle); "saturation" corresponds to whether a color is near the grey middle, or the vivid edge; "value" is also called "brightness" or "luminance," and is the third attribute.